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Gibbs Energy of Formation of Ammonia

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Gibbs Energy of Formation of Ammonia

Here, we calculate the Gibbs energy of formation of ammonia (NH3), given the reaction

 

N2 (g) + 3 H2 (g) → 2 NH3 (g)

 

Values of the Gibbs energy of formation tabulated in the literature are normally only given at standard temperature, or for a small number of temperatures. However, the ThermophysicalData:-Chemicals  package contains curve fits for experimental values of enthalpy and entropy of N2, H2 and NH3, correlated against temperature.

 

This means that Maple will help you calculate the Gibbs energy of formation of ammonia at any temperature (assuming that the enthalpy and entropy are within the bounds of the fitted data).

restart
with(ThermophysicalData:-Chemicals)

 

Temperature

T := 298.15*Unit('K')

 

Enthalpy

h_N2 := Property("Hmolar", "N2", "temperature" = T)

h_H2 := Property("Hmolar", "H2", "temperature" = T)
h_NH3 := Property("Hmolar", "NH3", "temperature" = T)

0.9915884626e-5*Units:-Unit(J/mol)

 

-0.4957942313e-5*Units:-Unit(J/mol)

 

-45940.00004*Units:-Unit(J/mol)

(1)


Entropy

s_N2 := Property("Smolar", "N2", "temperature" = T)

s_H2 := Property("Smolar", "H2", "temperature" = T)
s_NH3 := Property("Smolar", "NH3", "temperature" = T)

191.6097115*Units:-Unit(J/(mol*K))

 

130.6810143*Units:-Unit(J/(mol*K))

 

192.7702891*Units:-Unit(J/(mol*K))

(2)

 

Change in enthalpy and entropy per mole of NH3

DeltaH := .5*(2*h_NH3-h_N2-3*h_H2)
DeltaS := .5*(2*s_NH3-s_N2-3*s_H2)

-45940.00003*Units:-Unit(J/mol)

 

-99.05608805*Units:-Unit(J/(mol*K))

(3)

Hence the Gibbs Energy of Formation

DeltaG := -DeltaS*T+DeltaH

29533.57265*Units:-Unit(J/(mol*K))*Units:-Unit(K)-45940.00003*Units:-Unit(J/mol)

(4)

 

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